1. Field of the Invention
The present invention relates to the operation of communication systems and radar systems, and more particularly relates to methods and systems for providing a low probability of interception of transmissions.
2. Background of the Invention
The design of communication systems involves many interrelated issues. Two of the most important issues are privacy and multiple user accessibility. Private communication (or covert communication in military applications) is a goal sought to be accomplished by most communication systems. The ability to prevent unwanted parties from intercepting and eavesdropping on communications, intended to be private, is a major issue that must be considered during system design. Additionally, the ability for multiple users to have simultaneous access to the channel through which the communication system operates is another important issue. Accordingly, given this demand for multiple user accessibility, the related issue of privacy becomes even more important. Further, it should be noted that the same design considerations that provide for communication privacy also can reduce mutual interference among multiple users, which is a problem inherent in multiple user systems.
Thus, communication systems, designed to address these interrelated issues, have employed techniques for minimizing the probability that any particular data transmitted over a communications system will be intercepted by an unintended recipient. These techniques are collectively referred to as low probability of interception (LPI) techniques.
One such LPI technique known as spread spectrum modulation is often employed to attempt to ensure a private communication link. In general, spread spectrum modulation is a modulation technique in which the frequency bandwidth of the modulated signal (i.e., radio frequency signal) is, in effect, spread significantly beyond the frequency bandwidth of the modulating signal (i.e., information signal). Further, the bandwidth of the modulated signal is not dependent upon the bandwidth of the modulating signal. In effect, the spread spectrum technique is a coded pulse compression technique which uses time and frequency domain conversion in an integration process which is coherent only for authorized listeners with compatible receiving equipment. Two methods of providing the desired bandwidth spread associated with spread spectrum modulation are known as direct sequence and frequency hop. Different variations of the direct sequence and frequency hop techniques are also employed, as well as combinations thereof
The direct sequence spread spectrum technique involves multiplying a signal, modulated by some coherent digital technique such as phase shift keying, with a spreading code. The spreading code has the correlation properties of a pseudo-noise sequence, that is, a noise-like sequence that is actually deterministic in nature. At the receiver end of the communication system, ideally, the spreading code is available and is also time-synchronized with the code used to spread the signal. Thus, the signal is "unspread", or returned to its narrow bandwidth, while any interference or noise picked up during transmission remains spread and, thus, effectively suppressed. The signal, which was originally modulated by a coherent digital technique, such as phase shift keying, is then demodulated in the appropriate manner.
The frequency hop spread spectrum technique essentially involves "hopping" the modulated signal in a pseudo-random manner within a set of frequencies. Again, a signal, modulated in some manner, is mixed with a hopping code at the transmitting end and then mixed with a local oscillator offset in frequency by a synchronized hopping code at the receiving end to regenerate the original signal. Because the modulated signal is pseudo-randomly frequency hopped, an unwanted eavesdropper does not know which frequency band to monitor and thus must monitor the full frequency bandwidth in which the signal is hopped. By having to do so, the eavesdropper is faced with the disadvantage of dealing with the entire noise spectrum associated with this full frequency bandwidth.
However, while these spread spectrum techniques attempt to lower the probability of interception or jamming of data being transmitted by a communication system, these techniques, like all other existing LPI techniques, suffer from certain disadvantageous limitations. For instance, the processing gain of existing LPI techniques is limited by practical considerations. Processing gain is a quantification of the bandwidth disadvantage suffered by the eavesdropper as a result of spectrum spreading. Processing gain is equal to the ratio of spread bandwidth to despread bandwidth. This ratio is called processing gain because the ratio is also equal to the gain in the ratio of signal to noise plus interference which results from coherent processing to despread. Sometimes one-tenth of the common logarithm of this ratio is used to specify processing gain in dB. In the case of spread spectrum modulation, the processing gain is a function of the number of chips that can be summed. A chip refers to a discrete spreading code sequence. An attempt to increase the chip number, in an effort to increase processing gain, excessively limits the bandwidth of the data transfer because of the corresponding increase in integration (i.e., summation) time. Likewise, if the integration time is held constant and the number of chips is increased, the system encounters problems because the phase and timing tolerances are too stringent due to the individual chip lengths being too short. Thus, as is evident, the fundamental limitation associated with common LPI techniques, such as spread spectrum, is that the processing gain is directly related to the integration time of a single summation and, thus, the inherent disadvantage associated with attempting to increase the length of that individual summation.
Some radar systems also are designed to minimize the probability that the beams they use to illuminate targets will be detected by receivers other than the one used by the radar system to detect target echos. Spread spectrum LPI techniques are often employed for this purpose. However, the use of spread spectrum in a radar system is subject to the same processing gain limitations as in a communication system.
As a direct result of this limited processing gain, an eavesdropper can intercept data. The area of range in which an eavesdropper can intercept data includes the area between the transmitter and a minimum range equivalent to a fraction of the maximum transmission range of the communication system. This fraction is the reciprocal of the square root of the processing gain (the latter expressed as a bandwidth ratio rather than in dB). Thus, despite the application of existing LPI techniques, data transmissions may be intercepted in this region between the transmitter and this minimum range.